The benefit of upsizing to 4 gauge is slightly less voltage drop (power loss). Both the HPWC and 60A breaker can handle 4 gauge, so the larger wire doesn't cause any issue there. At 48 amps, it will create about 29 watts of heat in the 25' loop (50' of wire) with 4 gauge and 44 watts with 6 gauge.

I drop around 4 volts when charging at 48 amps. Thats means I'm wasting ~192 watts in heat. That's with ~65' of 2/0 Al wire feeding my garage panel (~48 watts at 48 amps), and ~35' of 4 gauge copper feeding my HPWC (~41 watts at 48 amps). Much of the noticeable heat is in the cord between the HPWC and the car, and in the breaker (high load breakers get warm).

I have an 18' HPWC cord, but could have used the short one. I suspect that's the majority of the remaining ~100 watts of loss in my circuit; it gets quite warm. If you want to conserve every watt, using the short cord HPWC is where I'd start.

For wire resistance, see:

http://www.paigewire.com/wire_resistance-prop.aspx?AspxAutoDetectCookieSupport=1